[llvm] [LV] fix logical error in trunc cost (PR #91136)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -7123,16 +7117,15 @@ LoopVectorizationCostModel::getInstructionCost(Instruction *I, ElementCount VF,
       // "zext i8 %1 to i32" becomes "zext i8 %1 to i16".
       //
       // Calculate the modified src and dest types.
-      Type *MinVecTy = VectorTy;
       if (Opcode == Instruction::Trunc) {
-        SrcVecTy = smallestIntegerVectorType(SrcVecTy, MinVecTy);
+        SrcVecTy = smallestIntegerVectorType(SrcVecTy, VectorTy);
         VectorTy =
-            largestIntegerVectorType(ToVectorTy(I->getType(), VF), MinVecTy);
+            smallestIntegerVectorType(ToVectorTy(I->getType(), VF), VectorTy);
----------------
artagnon wrote:

See the beginning of the function:

```cpp
InstructionCost
LoopVectorizationCostModel::getInstructionCost(Instruction *I, ElementCount VF,
                                               Type *&VectorTy) {
  Type *RetTy = I->getType();
  if (canTruncateToMinimalBitwidth(I, VF))
    RetTy = IntegerType::get(RetTy->getContext(), MinBWs[I]);
```

... and then VectorTy gets set here:

```cpp
  if (isScalarAfterVectorization(I, VF)) {
    // With the exception of GEPs and PHIs, after scalarization there should
    // only be one copy of the instruction generated in the loop. This is
    // because the VF is either 1, or any instructions that need scalarizing
    // have already been dealt with by the time we get here. As a result,
    // it means we don't have to multiply the instruction cost by VF.
    assert(I->getOpcode() == Instruction::GetElementPtr ||
           I->getOpcode() == Instruction::PHI ||
           (I->getOpcode() == Instruction::BitCast &&
            I->getType()->isPointerTy()) ||
           hasSingleCopyAfterVectorization(I, VF));
    VectorTy = RetTy;
  } else
    VectorTy = ToVectorTy(RetTy, VF);
```

https://github.com/llvm/llvm-project/pull/91136